Many wireless communication networks are based on a contention protocol similar to the Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) contention protocol. This protocol is used when a wireless station which is about to transmit communications, listens to the network (senses the carrier) before transmitting the communications and waits for an opportunity during which it may transmit the communications. The wireless station will not transmit as long as a packet is either currently detected in the air or air time was reserved by another wireless station for a predefined duration of time (virtual carrier sense). According to IEEE 802.11, the wireless station awaits a random period of time and then, if the air resource becomes available during that period, transmits the communications. If the receiver gets the frame intact, it sends back an ACK or a block ACK message (according to the request from the transmitter) to the sender, indicating that the packet has arrived. This protocol is mainly used for multiple-access and significantly reduces the possibility of contention, i.e. that two or more wireless stations will be transmitting at the same time.
However, a known problem in wireless networks that use the CSMA/CA protocol is the problem of the “hidden node” (also known as the “hidden terminal” problem). This problem addresses cases where a plurality of wireless stations (also referred to as mobile subscribers, mobile terminals, mobile, stations etc.) which are connected to the same access point, do not “hear” each other and as a result, two or more stations may transmit at the same time thereby causing a collision. Consequently, the Access Point (AP) is unable to detect neither one of the signals, causing them to retransmit after a random period—in which case collision may still happen again. The problem increases along with the increase in the number of wireless stations.
Some suggestions were made in the art to address the “hidden node” problem. The IEEE 802.11 protocol for example, uses a feature called RTS/CTS mechanism. According to this mechanism, each wireless station that is about to transmit a communication, sends a short Request-To-Send (RTS) packet to its respective access point and the access point returns a Clear-To-Send (CTS) packet. Both packets include the expected duration of the wireless transaction. All the other wireless stations detect the RTS packet or the CTS packet and create virtual carrier sense, which prevents them from accessing the air while the data transaction is still being transmitted.
Unfortunately, simple indoor wireless stations, such as laptops and smart phones, do not always have the support for transmitting the RTS messages. This is mainly because using the RTS/CTS type of negotiation, introduces overhead and decreases total system performance at times when no “hidden node” problem exists.
In addition, in high data rates packets introduced by future 802.11 standards (such as 802.11n, 802.11ac, 802.11ad), the length of the RTS packet may turn out to be longer than regular data packets. In these cases, transmitting RTS packets is not efficient and as a result many 802.11 stations use thresholds for deciding whether to transmit an RTS packet or not. Even in cases where RTS/CTS packets are being used, collisions and “hidden node”, problem might also occur in cases of collisions between the RTS packets themselves or between an RTS packet and another packet.